Sunan an-Nasa'i, Volume 1, Book 1, Hadith 310 (DarusSalam)

It was narrated from Abu Hurairah that the Prophet said: "When any one of you prays, let him not spit in front of him or to his right, rather let him spit to his left or beneath his feet." Then the Prophet spat like this on his garment and rubbed it.

أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، عَنْ مُحَمَّدٍ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ سَمِعْتُ الْقَاسِمَ بْنَ مِهْرَانَ، يُحَدِّثُ عَنْ أَبِي رَافِعٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ إِذَا صَلَّى أَحَدُكُمْ فَلاَ يَبْزُقْ بَيْنَ يَدَيْهِ وَلاَ عَنْ يَمِينِهِ وَلَكِنْ عَنْ يَسَارِهِ أَوْ تَحْتَ قَدَمِهِ وَإِلاَّ ‏" ‏ ‏.‏ فَبَزَقَ النَّبِيُّ صلى الله عليه وسلم هَكَذَا فِي ثَوْبِهِ وَدَلَكَهُ ‏.‏

sunnah.com

also referenced under: Book 1, Hadith 311 (In-Book)